Whisker deposits

This form of crystal growth differs from that of dendrites in that (a) it tends to have a still larger ratio between the longitudinal and the lateral dimensions with an almost perfect preservation of the latter during growth, and (b) it exhibits no tendency to side branching. Impurities or additives in the electrolyte seem to be a prerequisite for its appearance [11, 119]. 

Gorbunova et al. [120,121] grew silver whiskers firom fairly concentrated silver nitrate solutions ( 0.30 M) containing oleic acid, gelatin, albumin, heptyl, octyl, and nonyl alcohols. 

2 Mechanisms of Formation of Some Forms of Electrodeposited Pune Metals 

Finally, it should be noted that whiskers differ from other crystals of the same metal in two respects at least they have an increased electrical resistivity (two to three times that of crystals deposited in the absence of additives) and an increased tensile strength compared to large, pure silver single crystals [11]. 

A model of the growth mechanism was developed by Price et al. [119] which gives a good account of most of the phenomena observed. The basic assumption of the model is that molecules of impurities or additives are strongly adsorbed at aU but one crystal plane and at such a concentration as to completely block the 

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Ultrapure Al powder was used as a raw material. The Al powder was continuously fed in an N2 stream from the lower end of a reactor made of alumina. A gas mixture of NH3 and N2 was introduced in the reactor through another feeding tube. The nitridation temperature was controlled from 1450 to 1550°C. After nitridation, the whiskers deposited on the walls of the reactor and on the feeding tube of the gas mixture were collected. 

When the discharge current is large, delocalized pits formed in the anode are shallow, so the deposited lithium whiskers can easily emerge from the pits and stack pressure can be applied to them, as mentioned in Sec.3.7.3. 

Lackey, W., Hanigofsky, J., and Freeman, G., Experimental Whisker Growth and Thermodynamic Study of the Hafnium-Carbon System for Chemical Vapor Deposition Applications, 7] Amer. Ceram. Soc., 73(6) 1593-98 (1990)... 

Futamoto, M., Yuito, I, andKawabe, U., Hafnium Carbide and Nitride Whisker Growth by Chemical Vapor Deposition, /. Cryst. Growth, 61(l) 69-74 (Jan./Feb. 1983)... 

Glass OTC (GC) drawing 134 etching HCl 140 leaching HCl 140 particle deposition 143 properties 135 whisker formation 142 Glassy carbon (GC) 204 Golay equation GC 21 LC 342 SFC 607... 

Surface modification reactions are used to improve the wettability of glass surfaces by polar stationary phases and to Improve the extent of deactivation by sllylation" [138-146,166]. Miaaiuua procedures have been investigated but only a few are in use. Of these, the most important reactions are etching by hydrogen chloride, leaching with aqueous hydrochloric acid, formation of whiskers and solution deposition of a layer of solid particles. Because of the high purity and thinness of the... 

The crystalline NaCl-deposits formed outside of the Knudsen cells showed different morphology. Besides bulk crystals, mainly dendrites but also whiskers were formed. 

Alternatives to activated tungsten wire emitters are also known, but less widespread in use. Cobalt and nickel [44,47] as well as silver [48] can be electrochemi-cally deposited on wires to produce activated FD emitters. Mechanically strong and efficient emitters can be made by growing fine silicon whiskers from silane gas on gold-coated tungsten or tantalum wires of 60 pm diameter. [45] Finally, on the fracture-surface of graphite rods fine microcrystallites are exposed, the sharpness of which provides field strengths sufficient for ionization. [49]... 

In field ionization (as an experimental configuration) field ionization (the process) is the major pathway of ion generation. In field desorption from activated emitters, the analyte may also undergo field ionization. Presuming that the molecules are deposited in layers on the shanks of the whiskers or between them, this requires that i) analytes of low polarity are polarized by action of the electric field, ii) become mobile upon heating, and iii) finally reach the locations of ionizing electric... 

Direct metal deposition from metallic sources has been extensively used for model catalyst deposition for high-throughput and combinatorial studies. However, these methods are also increasingly used to deposit practical electrocatalyst materials. The best known approach is the one developed by 3M researchers have used physical vapor deposition to deposit Pt and Ft alloys onto nanostructured (NS) films composed of perylene red whiskers. The approach has been recently been reviewed by Debe. ... 

The nanostructured thin-film electrode was first developed at 3M Company by Debe et al. [40] and Debe [41], who prepared thin films of oriented crystalline organic whiskers on which Ft had been deposited. The film was then transferred to the membrane surface using a decal method, and a nanostructured thin-film catalyst-coated membrane was formed as shown in Figure 2.10. Interestingly, both the nanostructured thin-film (NSTF) catalyst and the CL are nonconventional. The latter contains no carbon or additional ionomer and is 20-30 times thinner than the conventional dispersed Pt/ carbon-based CL. In addition, the CL was more durable than conventional CCMs made from Pt/C and Nation ionomer [40]. 

Surface treatments of carbon fibers can in general be classified into oxidative and non-oxidative treatments. Oxidative treatments are further divided into dry oxidation in the presence of gases, plasma etching and wet oxidation the last of which is carried out chemically or electrolytically. Deposition of more active forms of carbon, such as the highly effective whiskerization, plasma polymerization and grafting of polymers are among the non-oxidative treatments of carbon fiber surfaces. 

Fuchs (1971) identified acicular wollastonite from the Allende meteorite. Miyamoto et al. (1979) called these extraterrestrial crystals whiskers. Massive deposits of fibrous wollastonite in minable quantities occur in the Adirondack Mountain region of New York State. 

Unfortunately, it has not been possible to suppress or control the whisker formation, to date. The whisker formation clearly results from the low thermodynamic stabihty of the adducts in the gas phase toward dissociation, which is the initial step of the whisker formation. Temperature-dependant deposition studies clearly showed an influence of the substrate temperature on the whisker growth at lower temperature the growth of single crystallites is preferred, whereas at higher temperatures, uncontrollable whisker growth takes place as can be seen from Fig. 10. 

Scholz and Hasse [377] have shown that crystals of metallic silver can be grown at liquid/liquid interfaces (water with AgNOs -octanol with ferrocene). Depending on the reactants concentrations in both phases, silver crystallized in the form of (1) wires (whiskers with radii from about 50 nm to 50 pm) growing into the organic phase (2) Christmas treelike deposits at the interface and even (3) extremely smooth films of silver. 

Whiskers have been produced in a range of fiber sizes and in three forms grown wool, loose fiber, and felted paper. The wool has a fiber diameter of 1-30 p,m and aspect ratios of 500-5000. The wool bulk density is about 0.03 g/cm, which is less than 1% solids. This is a very open structure, suitable for a vapor-deposition coating on the whiskers or direct use as an insulation sheet. Loose fibers are produced by processing the larger diameter fiber to yield lightly interlocked clusters of fibers with aspect ratios of 10-200. In felt or paper, the whiskers are randomly oriented in the plane of the felt and have fiber aspect ratios ranging from 250 to 2500. The felted paper has approximately 97% void volume and a density of 0.06-0.13 g/cm. ... 

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